Landing gear shock struts have been conventionally made of forged steel and comprise lugs for attachment of other components such as rods, arms, stays and braces (collectively referred to herein as “braces”) that transmit axial and bending forces and/or torque through to the cylinder of the shock strut. Efforts have been made to reduce the weight of aircraft struts and the use of lighter-weight materials such as titanium and aluminium has been developed. However these materials do not offer the step change in weight that carbon fibre composites do.
Composites are being used increasingly in applications to reduce the weight of structures. They are being employed to greater extents in aircraft landing gear components, replacing metal items such as braces, etc. It would be desirable to reduce landing gear weight further and make a shock strut cylinder from composite materials to reduce the weight of this part.
Composite shafts and other cylindrical members are typically formed by winding filaments such as carbon fibres around a mandrel in a helical fashion so as to build up a tubular shape. The filaments may be braided prior to being laid helically on the mandrel. The angle of the helical filaments influences the properties of the shaft and usually the shaft will comprise several layers of filaments wound at different winding angles. Various developments have been made to couple such composite shafts with metallic end fittings, i.e., to connect the orthotropic tubular structures with fittings having generally isotropic properties.
US 2012/0060636 A1 describes a clamping-type joint in which the composite fibres at the end of a shaft are trapped between a nut located inside the composite tube and an external collar that surrounds the tube of composite. A threaded end fitting component can be used to move the nut and the collar closer together, thus clamping the strut fibres therebetween.
US 2006/0258469 A1 describes a method of forming a composite shaft such that an internally splined end fitting can be axially press fit over the shaft resulting in good torque load transfer properties. US 2008/0012329 A1 describes a particularly advantageous spline profile for this end fitting.
In addition to supporting axial forces from the weight of an aircraft during take-off and landing, an aircraft landing gear shock strut must also withstand significant bending forces due to transverse loads transferred through braces, as well as some torque loads to either prevent the wheels from twisting undesirably from the intended direction during such manoeuvres or to enable the landing gear to steer the aircraft when required. On a metallic landing gear strut, lugs and other attachment features that are positioned midway along the strut can be incorporated into the casting or forging. However, when making the strut of composite materials, a problem arises of how to transmit and distribute axial forces, side loads and/or torque from the other components of the landing gear into the cylinder of the strut at points midway along the strut.
Similar considerations apply to other fields when struts that are generally subject to substantial compressive or tensile loads are being replaced by composite structures and where other force-transmitting components are required to be attached midway along the strut.